Authenticating pam and and sql

When a sql user logs in, dovecot always tries pam first (used for the local users with home directories) which generates a login failure in the log, before trying sql (virtual users) and allowing the user to login.

Since all the pam users login as 'user' and all the sql users login as 'user@example.com' is it possible to tell dovecot which method to check based on the username pattern?

It's not a big deal, but these errors show up when I am checking my maillog for problems, and I can't really exclude them since I do want to see actual login failures.